1. Motivation
The present invention relates generally to the field of composites and in particular to a new and useful consolidation and curing of composite parts by thermal pressing using a heated rigid tool and matched rubber tool.
The term “advanced composites” is generally understood to mean a polymer matrix reinforced by high-strength, high-modulus fibers of a predetermined orientation [Ref. 1—Rufe, P. D. (Editor), Fundamentals of Manufacturing, 2nd Ed., Society of Manufacturing Engineers, 2002]. Advanced composites use very stiff and strong, yet lightweight fibers of glass, carbon/graphite, Kevlar® (a registered trademark of E.I. du Pont de Nemours and Company for aramid fibers), boron, other organic materials and hybrid fiber mixes that are in woven, unidirectional, or continuous strand mat form. The polymer resin or “matrix” that holds the fibers together and transfer load can either be thermoset (e.g. epoxy, phenolic, bismaleimide and polyimide) or thermoplastic (e.g. nylon, polyester, polysulfone, polyphenylene sulfide, and PEEK). Composites that consist of layers bonded together are referred to as laminates, whereas a structure consisting of a low-density core (e.g. foam, honeycomb) between thin composite faces is called a sandwich.
The use of advanced composite materials has grown and continues to grow steadily, because their structure and processing can be tailored to applications requiring high strength, high stiffness, low weight, and/or low thermal conductivity. Typical products where advanced composite parts are used include aerospace structures (e.g. Boeing 787 Dreamliner, F-35 Joint Strike Fighter), automobiles and trucks (e.g. Chevrolet ZR1 Corvette, U.S. Army's HEMTT A3 tactile wheeled vehicle), spacecraft (e.g. SpaceShipOne), energy production (e.g. wind turbines), marine vessels (e.g. U.S. Navy's All-Composite Littoral Combat Ship), prosthetic devices, sports equipment (e.g. bicycle frames), medical devices, civil engineering structures, and many others. Looking at the growth rate of the carbon fiber and carbon fiber reinforced composites market alone, it has been about 12% for the last 23 years and the market size for industrial (including aerospace) and sporting goods made of carbon composite is slated to grow from over $7 billion in 2007 to $12.2 billion by 2011 [Ref. 2—Composite Application Market Assessment—A Global Overview, Frost & Sullivan (www.frost.com), published May 13, 2008].
Although composite materials are typically more expensive than most engineering metals and their alloys by weight (e.g. ˜$15-50/kg for aerospace-grade carbon/epoxy prepreg vs. >$1/kg for structural steel), the major problem limiting their use in products and subassemblies is arguably manufacturing time and expense. For example, a typical sequence of steps for high-performance thermoset composite manufacturing includes removal of thermoset prepreg roll from cold storage, cutting individual layers using a CNC cutter, hand working the manually heated layers into an open mold (known as “hand layup”) or over a core to form a laminate or sandwich, vacuum bagging the uncured composite over a dedicated mold, curing for hours it in an autoclave under high heat and pressure, debagging the composite workpiece, trimming and post machining in final geometrical features, inspection and finally fastening or bonding the final part to another structure [Ref. 3—Website link: http://www.compositesworld.com/articles/nacelle-manufacturers-optimize-hand-layup-and-consider-closed-molding-methods.aspx?terms=%40pub_CW_type+%3d++Feature].
Certainly not all composite systems and parts require each of the aforementioned labor, time, and energy-intensive manufacturing processes, but the more challenging applications do, particularly where weight reduction and performance are the driving factors such as with aerospace parts. The major issues with composites manufacturing are highlighted in a recent issue of Aerospace & Defense Manufacturing magazine, where several articles written by industry people and experts identify hand layup and autoclaving as the two major process bottlenecks, especially for the network of suppliers to major aerospace companies [Ref. 4—Bullen, G. N., “Get Rid of Those Autoclaves!” Manufacturing Engineering (Society of Manufacturing Engineers), Vol. 140, No. 3, March 2008 and Ref. 5—Morey, B., “Automating Composites Fabrication,” Manufacturing Engineering (Society of Manufacturing Engineers), April 2008 Vol. 140 No. 4, April 2008]. With regards to curing the composite, which is the focus of this patent application, one author went as far to say that “eliminating autoclaves is the ‘Holy Grail’ of composite manufacturers” [Ref. 6—Aronson, R., “Composites & Superalloys Fill Aerospace Needs,” Manufacturing Engineering (Society of Manufacturing Engineers), Vol. 140, No. 3, March 2008].
2. The Prior Art
Thermoset composite parts made by either wet layup or prepreg layup are typically vacuum bagged to remove air (called debulking), before and during the curing process. A typical vacuum bagging layup is shown in FIG. 1 [Ref. 7—Website link: http://media.photobucket.com/image/vacuum%20bagging%20composites/ebayPCI/VacuumBagLayup.jpg, accessed on Nov. 10, 2009].
The most common method for consolidating and curing a vacuum-bagged composite laminate is by using an autoclave. An autoclave is a pressure vessel that allows simultaneous application of vacuum to the bagged part, the application of external pressure to the outside of the bag to provide higher differential pressure, and heat to raise and hold the laminate's temperature to that level recommended by the material manufacturer [Ref. 8—Strong, A. B., Fundamentals of Composites Manufacturing: Materials, Methods, and Applications, 2nd Edition, Society of Manufacturing Engineers, Dearborn, Mich., 2008]. An inert gas (generally Nitrogen) is used in an autoclave to prevent oxidation of any components and explosions. A vacuum-bagged laminate such as that illustrated in FIG. 1 is loaded into an autoclave.
Consistent with the strong feelings against autoclaving by the composites industry, there has been some research and development devoted to eliminating the autoclave process step altogether. For example, Blair [Ref. 9—Blair, Michael. Composites: Success, Opportunity and Challenge. Composites Manufacturing Conference, 2007, Society of Manufacturing Engineers, Apr. 12, 2007] mentions efforts to lay and bond thermoplastic prepreg tape using ultrasonics and matrix materials which cure when exposed to ultraviolet radiation. Other researchers have looked at using electron beams, gamma rays and microwaves for curing composites [Ref. 10—J N Hay, J. N. and O'Gara, P., 2006, “Recent developments in thermoset curing methods,” Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, Vol. 220, No. 3, pp. 187-195]. However, a more practical approach to address immediate industry needs is to develop alternative composite curing processes that work for existing thermoset matrix materials already on the market.
Tubular thermoset composite parts, such as a tennis racquet, require internal pressurization by wrapping the prepreg material around a bladder, placing the assembly into a heated “clamshell” mold, and pressurizing the bladder to force the wound laminate against the inner mold surfaces until cured [Ref. 8 above]. This is called “bladder molding,” and a schematic of the basic process is shown in FIG. 2. [Ref. 11-Website: http://www.carbonbydesign.com/features/features_bladder.asp, accessed on Nov. 10, 2009].
The exact opposite of bladder molding is consolidating and curing a tubular thermoset composite part using external pressure from a fluid. A patent by Park [Ref. 12—Park, J. F., “Method and System for Curing Fiber Reinforced Composite Structures,” U.S. Pat. No. 5,643,522, 1997] discusses a long annular-shaped bladder contained within a cylindrical pressure vessel. Long prepreg composite parts would be inserted within the annulus, the ends are closed with endcaps, and heated pressurized fluid circulating through the bladder envelopes the composite part to cure it. Rapid changes in temperature are achieved by a fluid control system which can divert fluid maintained at three different temperature ranges in three large tanks. This eliminates the need to thermally cycle a single fluid reservoir.
A very similar apparatus and method for curing composites to that of Park was invented by Graham [Ref. 13—Graham, N., “Method of Manufacturing Composites,” U.S. Pat. No. 6,149,844, 2000]. In this apparatus, one side of the prepreg laminate is guided by a flexible diaphragm material. On the other side of the laminate is a shaped tool floating on another flexible diaphragm, which is used to impart the required surface shape and finish to the composite. The prepreg part to be cured is placed in direct contact with the mold after it is sprayed with a release material. Heated and pressurized Heat Transfer Fluid (HTF) supplied behind both flexible diaphragms sandwiches the tool and prepreg laminate to cure it.
By including two or more sources of HTF at different temperatures, the curing chamber and thus the laminate being cured can be heated and subsequently cooled more quickly. Also, by maintaining these respective fluids in large reservoirs, the need to thermally cycle the temperature of a single fluid chamber is eliminated, thus increasing the energy efficiency of the system. This technology has proven to be extremely successful and is commercially available through Quickstep Technologies Ltd. [Ref 14—Website: http://www.quickstep.com.au/what-is-quickstep/how-quickstep-works, accessed on Sep. 20, 2008]. Quickstep also vibrates the HTF and draws vacuum between the diaphragms early in the curing cycle to remove entrapped air as shown in FIG. 3 [Ref. 15—Website: http://compositecenter.org, accessed on Nov. 10, 2009].
Kemp [Ref. 16—Kemp, D. N. “Fixed-volume, trapped rubber molding method,” U.S. Pat. No. 4,889,668, 1989] patented a process called ‘fixed-volume trapped rubber molding,’ where the thermal expansion of a heated rubber mold that is constrained to a fixed volume provides the pressure and heat required to consolidate and cure a thermoset composite laminate part.
Another alternative to autoclaving is “prepreg compression molding” [Ref. 8 above]. A prepreg layup is placed by hand into the cavity of a heated match mold in the open position. The mold is closed bring the male and female mold halves together to exert pressure on the prepreg layup for further consolidation. After the part has cured, the mold halves separate and the part is removed. A schematic of the compression molding process for a 2-D part is shown in FIG. 4 [Ref. 17—Website: http://www.greenhulk.net/forums/showthread.php?t=22098, accessed on Nov. 10, 2009]. Blackmore [Ref. 18—Blackmore, R., “Advanced Cured Resin Composite Parts and Method of Forming Such Parts,” U.S. Pat. No. 5,648,137, 1997 and Ref. 19—Blackmore, R., “Method of Forming Advanced Cured Resin Composite Parts,” U.S. Pat. No. 5,656,231, 1997] invented a compression molding system for curing composite parts, where each mold half contains a conductive layer that is used to provide resistive heating for curing prepreg composite parts. The molds must be constructed of a material with similar expansion and contraction coefficients to ensure that even pressure and temperature are provided.